The molecular structure of 1,2,3-trimethyldiaziridine has been determined from the gas-phase electron diffraction data supplemented spectral and quantum chemical calculations. The configuration of studied compound incorporates trans-position of methyl groups attached to nitrogen atoms of diaziridine cycle. The following principal structural parameters were determined (r h1 bond lengths in Å, bond angles in degrees with 3σ in parentheses): r(N-C), 1.489(9); r(N-N), 1.480(15); r(C-C), 1.503(15); ∠NCN, 61.5( 9); ∠(H 3 C)CN, 124.0(15). The obtained structural parameters of 1,2,3-trimethyldiaziridine were compared with those for structural analogues. The gaseous standard enthalpy of formation of 1,2,3-trimethyldiaziridine was estimated to be 176.2 ± 5.0 kJ/mol.